Toggle darkmode от MrOtherGuy

To edit the used colors go to Firefox Options ->Language&Appearance and hit the Colors... button. While there modify the background color to something dark and the Text color to something light. Might want to make unvisited link colors slightly lighter blue also.

Unfortunately Android doesn't expose these settings, but you can do the same thing via about:config.
Edit the prefs "browser.display.background_color" for background and "browser.display.foreground_color" for text.

All this extension does is is add a button to toggle the preference that tells Firefox whether to always or never use these override colors.